Queen Victoria was not at all amused: “We do not believe that anyone could be insane who wanted to murder a Conservative Prime Minister.” The target of a number of assassination attempts herself, she was upset enough upon hearing the verdict in the trial of Daniel M’Naghten that she complained to the Prime Minister, prompting the House of Lords to revive an ancient right to intervene by directing the Supreme Court of Judicature to respond to five questions about the case.

M’Naghten was a Scottish woodcutter who acted on delusions that he was being persecuted by the Conservatives by attempting to murder tory Prime Minister Robert Peel: “The Tories have … compelled me to do this. They follow, persecute me wherever I go, and have entirely destroyed my peace of mind ….”

Setting aside the queen’s implication about the political affiliation of the victim, the facts are well known. M’Naghten had exhibited symptoms of mental illness for at least a couple of years, his landlady reporting that he believed there were devils in human form seeking to kill him. Those devils evidently enjoyed the form of conservative politicians because some time later, he applied to the London Police for protection from the Tory persecutors who sought his life. On January 20, 1843, a few days after being seen loitering suspiciously near Whitehall, he mistook Edward Drummond for Peel, walked up to him in broad daylight, drew his gun, and shot him in the back. When brought for trial and asked for his plea, he asserted that he was “driven to desperation by persecution” and that he was “guilty of firing.” After testimony by a number of witnesses, including physicians who testified that M’Naghten’s delusions deprived him of control over his actions, he was found not guilty on the ground of insanity.

For many years, the M’Naghten test defined the circumstances under which a defendant could succeed in using insanity as a defense to a crime. The test developed in response to the five questions put to the Court of Judicature by the House of Lords: a person is not guilty if, at the time of the person’s actions, he either did not know the nature or quality of his actions or did not know what he was doing was wrong. In modern parlance, there are two tests, either of which may be sufficient for an insanity defense to succeed: the “cognitive capacity” test and the “moral capacity” test. But it is the response to the fifth question in particular that interests me today, in which the Court noted the impracticality of deciding the legal question of insanity with science alone; instead, “each of those questions involves the determination of the truth of the facts deposed to, which it is for the jury to decide, and the questions are not mere questions upon a matter of science.” The record of the case can be found here.

As Jared Loughner entered a plea of not guilty on Monday after opening fire in Tucson earlier this month, killing a federal judge and seriously injuring a member of Congress, many have questioned to what extent an insanity defense might apply. The parallels with the M’Naghten case are difficult to avoid: Loughner appears to have been suffering from a number of delusions, and irrationally focuses responsibility for his ills on the U.S. government. What is perhaps most surprising, though, is how little the view about the role of psychiatric science in bearing on the question of insanity has changed in 150 years. After all, the last few years have seen tremendous advances in understanding the structure of the human brain and the physiological relationship with many mental illnesses: the technique of functional MRI allows specific brain structures to be associated with different activities, different thoughts, different senses, etc.

Coincidentally, it was the State of Arizona’s insanity test that was the subject of the U.S. Supreme Court’s most recent ruling on the defense, the same state where Loughner went on his rampage. While Arizona had historically codified the full M’Naghten test, with both the cognitive-capacity and moral-capacity prongs, it dropped the cognitive-capacity test in 1993 so that a defendant must lack moral capacity for an insanity argument to succeed. There is a great deal of discussion of different issues related to insanity in the Court’s opinion in Clark v. Arizona (which can be read here), but of particular interest is its general distrust of evidence related to mental disease — and acceptance of Arizona’s restrictions on considering such evidence — even as recently as 2006:

[T]he diagnosis may mask vigorous debate within the profession…. [T]his professional ferment is a general caution in treating psychological classifications as predicates for excusing otherwise criminal behavior…. [T]here is the potential of mental-disease evidence to mislead jurors … through the power of this kind of evidence to suggest that a defendant suffering from a recognized mental disease lacks cognitive, moral, volitional, or other capacity, when that may not be a sound conclusion at all…. These dangers arise because of the imperfect fit between the questions of ultimate concern to law and the information contained in a clinical diagnosis.

The Court’s conclusion that a psychiatrist has no more particularly relevant knowledge on the legal and moral issues related to an insanity defense than does a layman has, unsurprisingly, been subject to significant criticism in psychiatric circles. The American Psychiatric Association and the American Psychological Association have both taken the position, seemingly reasonable, that those who study the human brain are in a position to offer expert evidence about how the functioning of the brain can lead individuals to take actions that we all consider horrific. A copy of their joint amicus brief in Clark can be read here. Indeed, it seems more that there is a widespread misunderstanding of mental disease that could be demystified through expert testimony rather than the Court’s view that such testimony may have the effect of clouding the issues and misleading jurors. Disagreement among experts is nothing new: juries are called upon all the time to evaluate disagreements, even among experts, so they can reach decisions in cases that are in some sense “fair.”

Contrary to widespread conception, insanity defenses are only rarely raised and even more rarely succeed. By all accounts, it appears that Loughner did indeed possess moral capacity when he opened fire, even if he was suffering from certain delusions: his statements on his Myspace page and his quick statement to “plead the Fifth” when he was arrested demonstrate a knowledge that he well understood what he did was wrong. I suspect that an insanity defense, if raised, will fail. But at the same time, as someone who generally believes that we make better decisions when we have more information rather than less, I’m uncomfortable with limiting the evidence of mental illness that a jury can consider when evaluating that defense.